1. Field of the Invention
The present invention relates to an optical amplifier for amplifying a wavelength division multiplexed (WDM) light containing a plurality of signal lights of different wavelengths, and in particular, to an optical amplifier provided with variable optical attenuators on inter-stages of a plurality of amplifying sections which are connected in series to each other.
2. Description of the Related Art
A demand for communications traffic is drastically increased with the development of multimedia networks, and a WDM transmission system for multi-repeatedly amplifying optical signals using optical amplifiers, has a large role for achieving the economization of communication system in the multimedia society.
A transmission loss assumed in the WDM transmission system is in a significantly wide range of about 0 to 30 dB. However, in the case where this transmission loss is compensated for by optical amplifiers of various types, there are caused problems of cost increases due to the stock of the various types of optical amplifiers, and the labor cost and time required in the selection of the optical amplifier types. Therefore, it is desired to compensate for the above transmission loss by the optical amplifiers of one type. If this desire is replaced with operating conditions of the optical amplifier, three performances, namely, the constant output power, flat gain wavelength characteristics and low noise figure (NF), to a broad input dynamic range, should be realized.
Further, the maximum number of wavelengths in the WDM transmission system is in wide range of, for example, 2 to 40 waves and so on, in response to the communication traffic volume. However, also in this case, in the same reason as the above, it is desired to cope with arbitrary number of wavelengths by optical amplifiers of one type. In such a case, in the optical amplifier, since a gain per one wavelength can be made large as the number of wavelengths is smaller, the favorable NF can be obtained as the number of wavelengths is smaller. As a result, in the WDM transmission system in which the maximum number of wavelengths is small, it becomes possible to achieve an increase of the number of repeating stages.
As a conventional technology for realizing the performances required for the optical amplifier used in the WDM transmission system as described above, there has been known, for example, a configuration in which a variable optical attenuator (VOA) which is seemingly contradictory to the amplification is arranged between a former stage amplifying section and a latter stage amplifying section in an optical amplifier of two staged configuration, and this configuration is a standard configuration of the optical amplifier in the WDM transmission system (refer to Japanese Patent No. 3551418).
Such a conventional optical amplifier applies a control method in which the total power of an input/output WDM light is monitored to thereby control a gain in each optical amplifying section to be constant, and also, an attenuation amount of the variable optical attenuator provided on an inter-stage of the amplifying sections is changed according to the fluctuation in a signal light input level, to thereby retain a predetermined signal light output level while maintaining the flatness of output wavelength characteristics.
However, in the conventional optical amplifier in which the variable optical attenuator is arranged on the inter-stage of the amplifying sections, since a control is performed for increasing the attenuation amount of the variable optical attenuator in response to an increase of the signal light input level, there is a problem in that the NF is abruptly degraded in a region where the signal light input level is high.
In order to solve the above problems, the applicant of the present invention has proposed a configuration in which, in addition to the inter-stage variable optical attenuator, variable optical attenuators are arranged on an output side of the latter stage amplifying section and further, on an input side of the former stage amplifying section, so that the respective amplifying sections are automatically gain controlled, and at the same time, the attenuation amount of each of the variable optical attenuators is interconnected controlled according to the signal light input level per one wavelength of the WDM light (refer to Japanese Patent Application No. 2005-252508).
To be specific, in the configuration in which the variable optical attenuators are arranged on the inter-stage of the former stage amplifying section and the latter stage amplifying section, and on the output side of the latter stage amplifying section, when the signal light input level per one wavelength of the WDM light is lower than a previously set base point level, the WDM light input to an input port is amplified by the former stage amplifying section which is automatically gain controlled, and thereafter, passes through the inter-stage variable optical attenuator of which attenuation amount is set at minimum, and is sent to the latter stage amplifying section which is automatically gain controlled to be amplified, and further, is attenuated by the output side variable optical attenuator of which attenuation amount is controlled according to the signal light input level, so that the WDM light of which signal light output level is controlled to be constant, is output from an output port. On the other hand, when the signal light input level is higher than the base point level, the WDM light amplified by the former stage amplifying section is attenuated by the inter-stage variable optical attenuator of which attenuation amount is controlled according to the signal light input level, and further, the WDM light amplified by the latter stage amplifying section is attenuated by the output side variable optical attenuator of which attenuation amount is controlled according to the signal light input level. As a result, since the attenuation amount required for the inter-stage variable optical attenuator is decreased with the application of the output side variable optical attenuator, it becomes possible to suppress the deterioration of the NF in the region where the signal light input level is high.
Incidentally, in the configuration in which the variable optical attenuator is also arranged on the input side of the former stage amplifying section, even in the case where the signal light input level exceeds a level capable to be coped with by the inter-stage and the output side variable optical attenuators, a further broad input dynamic range is realized, by attenuating to a constant level the power of the signal light fed to the former stage amplifying section by the input side variable optical attenuator.
However, in the optical amplifier proposed in the prior invention as described above, there is caused a problem in that, although the desired NF can be obtained in a state where the number of wavelengths of the WDM light is large (for example, 40 waves and so on), it is hard to obtain the desired NF in a state where the number of wavelengths of the WDM light is small (for example, 2 waves and so on). Namely, in the configuration of the prior invention, since the respective former stage amplifying section and the latter stage amplifying section are automatically gain controlled, gains of the respective amplifying sections are controlled to have the same value, irrespective of the number of wavelengths of the WDM light. Therefore, in the state of the small number of wavelengths, each amplifying section which has the performance capable of obtaining a required gain when the number of wavelengths of the WDM light is maximum, is operated without exerting the intrinsic performance thereof, and based on such an operating state of each amplifying section, a control target value of the attenuation amount of each of the inter-stage and output side variable optical attenuators is decided. Accordingly, in the case where not only the signal light input level per one wavelength of the WDM light but also an increase/decrease of the number of wavelengths is considered, there is room for improvement of the NF in the prior invention.
Further, as a separate problem from the NF, in the configuration in which the variable optical attenuator is arranged on the output side of the latter stage amplifying section, in order to realize a desired signal light output level, it is required to obtain a higher gain in the latter stage amplifying section. Thus, there is caused the necessity to apply a pumping light source of large power to the latter stage amplifying section, resulting in drawbacks of an increase of power consumption, the degradation in heat dissipation, the rise of manufacturing cost and the like.